Rocks that fold at low temperatures and pressures
tend to fold like this - the layers bend like a stack of
rubber sheets and maintain fairly constant thickness.
Note that the layers in the center become contorted
because they lack room to move. West coast of Scotland
south of Glasgow.

More plastic rocks deform like these rocks on Gibbs
Island, Antarctica. Some rocks are highly plastic by
nature, but rocks of all kinds become plastic at high
temperatures and pressures. We can define the axial
plane of the folds as the surface that connects the
sharpest bends in each layer. Here the axial planes are
horizontal. Note that the thickness of the layers varies
tremendously, but the thickness measured parallel to
the axial plane is much more nearly constant.
Basically the layers have no mechanical significance; the
rock flows under pressure and the layers simply smear out, like smearing
striped cake icing with a knife.

Rocks with really complex histories often look like
this. Often there is no discernible pattern to the
folding.

When rocks fold, individual layers within the fold
often crinkle. These small folds tend to have S-shapes on
one side of the axial plane and z-shapes on the other.
The significance of these minor folds is that the
geologist can tell which side of a large fold she is on
and locate its axial plane by observing the shapes of the
small folds.

Using the shapes of minor folds, it's possible to
work out the structure of even intensely deformed rocks.
At first glance these rocks in the Scottish Highlands
look hopeless, but closer inspection shows that, as
intricately folded as the rocks are, they basically
define a syncline. The geologist on the outcrop figured
that out, and eventually zeroed in on a place where it
could be seen, by mapping the shapes of small folds in
the rocks.

To participants (survivors), the day this
photo was taken will live forever in memory as the Black
Friday Death March. To get here, we climbed 3,000 feet up
one mountain, 1,500 feet into a valley and 1,500 feet up
the other side. It is now 4 P.M. and we are 8-1/2 miles
from the nearest road.

Rocks can be folded more than once, as these gneisses
near Sudbury, Ontario were.

Kink Bands

Foliation

Deformed rocks develop a sheetlike structure called foliation.
one way foliation can form is simply by flattening
existing strucures in the rock. This rock near Gander,
Newfoundland was once a conglomerate - granitic pebbles
have been flattened by over 50 per cent but the black
slate pebbles have been flattened almost paper-thin.

Foliation is not bedding. This slate paving stone
near Independence Hall, Philadelphia was split along a
foliation surface, but weathering has revealed the
original bedding in the rock.

Some foliation forms when materials are removed from
the rocks by solution. The fine parallel cracks in this
fold formed that way. The solutions move in the direction
of least resistance, parallel to the axial plane of the
fold.

Boudinage

When rocks are compressed, they often are extended at
the same time in some other direction. The granite dike
cutting this gneiss near Bucksport, Maine has been
stretched until it broke into a series of discrete blobs,
a process called boudinage (from the French word
for sausage.)